In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
In WO2006/003077 a 12f reflection type coaxial holographic storage system with three confocally arranged Fourier planes is shown. In this arrangement the object beam and the reference beams are coupled in and out at the first and third Fourier planes, respectively. The reference beams are small spots in these planes. More precisely, they form diffraction patterns, similar to the Airy pattern. This arrangement is a so-called common aperture arrangement, because at the object plane and the image plane the object beam and the reference beams fill out the same area of the aperture. The beams fill out the entire aperture of the objectives. The disclosed arrangement allows to apply shift multiplexing, reference scanning multiplexing, phase coded multiplexing, or a combination of these multiplexing schemes. The reference beams are a pair (or pairs of) half cone shaped beams. The tips of the pair or pairs of half cone shaped reference beams form two lines along a diameter at the Fourier planes of the object beam.
In EP1918914 a holographic storage system using multiple reference beams has been proposed. Two or more spherical reference beams are arranged equally around of the low pass filtered Fourier plane of the spatial light modulator. The reference beams are spots, or more precisely Airy distributions, at the Fourier plane. In the storage material the reference beams act as cones with parallel axes, shifted relative to each other by the diameter of the Fourier filter. In order to avoid phase conjugated read out in reflection type arrangements half-cone reference beams are used instead of full cones.
Holographic storage systems suffer from a plurality of different noise sources. The main sources are the inter-pixel and inter-hologram cross talk, material scattering, detector noise, vibration and other environmental disturbances, servo misalignments, etc. In order to achieve a high data density and high data rates it is necessary to eliminate as many noise sources as possible.